PET device and image generating method for pet device

ABSTRACT

A PET apparatus is provided, which ensures excellent quantitativeness by properly correcting the influence of scattered radiation while improving the resolution of a reconstructed image and keeping good photon pair detection sensitivity. A determining section  52  determines whether a straight line connecting the light-receiving surfaces  15   b  of a pair of photon detectors  15   a  which have simultaneously detected a photon pair crosses any one of slice collimators  21   n . When it is determined that the straight line crosses none of the slice collimators  21   n , the corresponding coincidence counting information is accumulated by a first coincidence counting information storage section  53  to generate a signal sinogram. When it is determined that the straight line crosses one of the slice collimators  21   n , the corresponding coincidence counting information is accumulated by a second coincidence counting information storage section  54  to generate a scatter sinogram. An image reconstructing section  60  corrects the influence of scattered components on the signal sinogram on the basis of the scattered sinogram, and reconstructs an image on the basis of the corrected signal sinogram.

TECHNICAL FIELD

The present invention relates to a PET apparatus and an image generatingmethod for the PET apparatus which can image the behavior of a substancemarked by a positron emission isotope (RI radiation source).

BACKGROUND ART

A PET (Positron Emission Tomography) apparatus is an apparatus which canimage the behavior of a trace substance in an object (living body) to beexamined by detecting a pair of 511 keV photons (gamma rays) which flyin opposite directions upon electron-positron pair annihilation in theobject irradiated with RI radiation. The PET apparatus includes adetecting section having many small photon detectors arrayed around ameasurement space in which an object to be examined is placed. Thisapparatus detects a photon pair generated upon electron-positron pairannihilation by coincidence counting, accumulates the coincidencecounting information, and reconstructs an image representing the spatialdistribution of occurrence frequencies of photon pairs in themeasurement space on the basis of these many pieces of accumulatedcoincidence counting information. This PET apparatus serves an importantrole in the field of nuclear medicine and the like. For example,biofunctions and the high-order brain functions can be studied by usingthis apparatus. Such PET apparatuses are roughly classified intotwo-dimensional PET apparatuses and three-dimensional PET apparatuses.

FIG. 11 is a view for explaining the arrangement of the detectingsection of a two-dimensional PET apparatus. FIG. 11 shows an example ofan arrangement including seven detector rings, and is a sectional viewof the detecting section taken along a plane including the central axis.A detecting section 10 of the two-dimensional PET apparatus has detectorrings R₁ to R₇ stacked between a shield collimator 11 and a shieldcollimator 12. Each of the detector rings R₁ to R₇ has a plurality ofphoton detectors arranged in the form of a ring on a plane perpendicularto the central axis. Each photon detector is a scintillation detectorformed from a combination of a scintillator such as BGO (Bi₄Ge₃O₁₂) anda photomultiplier. This detector detects photons flying from ameasurement space including the central axis. The two-dimensional PETapparatus has slice collimators S₁ to S₆ inside the detecting section10. These slice collimators S₁ to S₆ are ring-like members each of whichis placed between adjacent detector rings in a direction parallel to thecentral axis. Each slice collimator is made of a material having alarger atomic number and larger specific gravity (e.g., lead ortungsten) and has a collimating function of shielding obliquely incidentphotons (gamma rays).

The detecting section 10 of the two-dimensional PET apparatus having theabove arrangement can perform coincidence counting of only a photon pairflying from the nearly 90° direction with respect to the central axisowing to the collimating function of the slice collimators S₁ to S₆.That is, the coincidence counting information, i.e., two-dimensionalprojection data, accumulated by the detecting section 10 of thetwo-dimensional PET apparatus is limited to that obtained by a pair ofphoton detectors included in a single detector ring or detector ringswhich are adjacent to each other (or very close to each other). Thetwo-dimensional PET apparatus can therefore efficiently remove scatteredradiation produced when a photon pair generated outside the measurementspace is scattered. In addition, this apparatus can easily performabsorption correction and sensitivity correction with respect totwo-dimensional projection data, and hence can obtain a reconstructedimage with good quantitativeness.

FIG. 12 is a view for explaining the arrangement of the detectingsection of the three-dimensional PET apparatus. FIG. 12 is also asectional view of the detecting section taken along a plane includingthe central axis. The arrangement of the detecting section 10 of thethree-dimensional PET apparatus is the same as that of thetwo-dimensional PET apparatus except that the three-dimensional PETapparatus has no slice collimators. The detecting section 10 of thethree-dimensional PET apparatus having this arrangement has a wide solidangle and can perform coincidence counting of a photon pair flying froma wide range as compared with the two-dimensional PET apparatus. Thatis, as the coincidence counting information, i.e., three-dimensionalprojection data, obtained and accumulated by the detecting section 10 ofthe three-dimensional PET apparatus, data obtained by a pair of photondetectors included in an arbitrary detector ring can be used.Three-dimensional PET apparatus can therefore perform coincidencecounting of a photon pair with sensitivity five to ten times higher thanthat of the two-dimensional PET apparatus. As compared with thetwo-dimensional PET apparatus, however, the three-dimensional PETapparatus has difficulty in accurately removing the influence ofscattered radiation, and hence the quantitativeness of a reconstructedimage is poor.

As described above, as compared with the three-dimensional PETapparatus, the two-dimensional PET apparatus having slice collimatorshas low photon pair detection sensitivity but can efficiently removescattered radiation and easily perform absorption correction andsensitivity correction. The two-dimensional PET apparatus therefore hasthe merit of obtaining a reconstructed image with excellentquantitativeness.

DISCLOSURE OF THE INVENTION

The above PET apparatuses, however, have the following problems. Boththe two-dimensional PET apparatus and the three-dimensional PETapparatus are required to improve the resolution of an image. In orderto improve the resolution, it is indispensable to reduce the size ofeach photon detector.

In the case of the two-dimensional PET apparatus, however, the intervalsbetween the respective slice collimators decrease with a reduction inthe size of each photon detector, and hence the open area ratiodecreases to result in a deterioration in photon pair detectionsensitivity. In the two-dimensional PET apparatus, a reduction in openarea ratio can be suppressed by thinning and shortening each slicecollimator in accordance with a reduction in the size of each photondetector. This, however, decreases the effect of shielding photons(gamma rays), i.e., the collimating effect, and hence scatteredradiation cannot be efficiently removed, resulting in a deterioration inthe quantitativeness of a reconstructed image.

In the case of the three-dimensional PET apparatus, even if the size ofeach photon detector is reduced, since no slice collimator is used, noproblem arises concerning a reduction in open area ratio or a decreasein photon pair detection sensitivity. As described above, however, sinceit is essentially difficult to eliminate the influence of scatteredradiation in the three-dimensional PET apparatus, the quantitativenessof a reconstructed image is poor.

The present invention has been made to solve the above problems, and hasas its object to provide a PET apparatus and an image generating methodfor the PET apparatus, which realize excellent quantitativeness byproperly correcting the influence of scattered radiation while improvingthe resolution of a reconstructed image and keeping photon pairdetection sensitivity high.

According to an aspect of the present invention, there is provided a PETapparatus characterized by comprising (1) a detecting section whichincludes a plurality of cylindrical detectors each formed bytwo-dimensionally arraying a plurality of photon detection elements,each of which detects a photon flying from a measurement space includinga central axis, on a cylinder surrounding the central axis, theplurality of cylindrical detectors being arrayed in a direction parallelto the central axis, (2) a plurality of slice collimators which arealternately arranged with the cylindrical detectors at least between themeasurement space and the detecting section in a direction parallel tothe central axis, and pass only photons, of photons flying from themeasurement space, which are substantially parallel to a predeterminedplane perpendicular to the central axis toward the detecting section,(3) a determining section which determines, when a pair of photondetection elements included in the detecting section simultaneouslydetect a photon pair, whether a straight line connecting light-receivingsurfaces of the pair of photon detection elements crosses any one of theplurality of slice collimators, (4) a first coincidence countinginformation accumulating section which accumulates coincidence countinginformation of the photon pair detected by the pair of photon detectionelements when the determining section determines that the straight linecrosses none of the plurality of slice collimators, (5) a secondcoincidence counting information accumulating section which accumulatescoincidence counting information of the photon pair detected by the pairof photon detection elements when the determining section determinesthat the straight line crosses one of the plurality of slicecollimators, and (6) an image reconstructing section which corrects aninfluence of a scattered component on the coincidence countinginformation accumulated by the first coincidence counting informationaccumulating section on the basis of the coincidence countinginformation accumulated by the second coincidence counting informationaccumulating section, and reconstructs an image representing a spatialdistribution of occurrence frequencies of photon pairs in themeasurement space on the basis of the corrected coincidence countinginformation.

According to the PET apparatus according to an aspect of the presentinvention, of 511 keV photon (gamma ray) pairs generated uponelectron-positron pair annihilation in a measurement space, a photonpair that has reached the detecting section without being shielded by aplurality of slice collimators are simultaneously detected by a pair ofphoton detection elements included in the detecting section. Thedetermining section determines whether a straight line connecting thelight-receiving surfaces of the pair of photon detection elements thathave simultaneously detected the photon pair crosses any one of theslice collimators. If the determining section determines that thestraight line crosses none of the slice collimators, the coincidencecounting information of the photon pair detected by the pair of photondetection elements is accumulated by the first coincidence countinginformation accumulating section. If the determining section determinesthat the straight line crosses one of the slice collimators, thecoincidence counting information of the photon pair detected by the pairof photon detection elements is accumulated by the second coincidencecounting information accumulating section. When a predeterminedmeasurement period comes to an end, the first and first coincidencecounting information accumulating sections stop accumulating coincidencecounting information. The image reconstructing section corrects theinfluence of scattered components on a signal sinogram accumulated andgenerated by the first coincidence counting information accumulatingsection, on the basis of a scatter sinogram accumulated and generated bythe first coincidence counting information accumulating section, andreconstructs an image representing the spatial distribution ofoccurrence frequencies of photon pairs in the measurement space on thebasis of the corrected signal sinogram.

In the PET apparatus according to one aspect of the present invention,detection of coincidence counting information may be performed by a pairof photon detection elements in the same cylindrical detector includedin the detecting section, a pair of photon detection elementsrespectively included in two adjacent cylindrical detectors depending onthe sizes of each cylindrical detector and each slice collimator, or apair of photon detection elements included in two separate cylindricaldetectors. In other words, detection of coincidence counting informationmay be performed between two adjacent detector rings or between twoseparate detector rings as well as within the same detector ring (onelayer of photon detection elements arrayed in the form of a ring in adirection parallel to the central axis). That is, the PET apparatusaccording to the present invention has an intermediate arrangementbetween a conventional two-dimensional PET apparatus and a conventionalthree-dimensional PET apparatus, and has sensitivity about several timeshigher than that of the conventional two-dimensional PET apparatus. ThePET apparatus according to the present invention can therefore ensuregood photon pair detection sensitivity and quantitativeness whileimproving the resolution of a reconstructed image.

In the PET apparatus according to one aspect of the present invention,in particular, the determining section determines whether a straightline connecting the light-receiving surfaces of a pair of photondetection elements which have simultaneously detected a photon paircrosses any one of the slice collimators. The first coincidence countinginformation accumulating section generates a signal sinogram on thebasis of this determination result. The second coincidence countinginformation accumulating section generates a scatter sinogram. The imagereconstructing section then corrects the influence of scatteredcomponents on the signal sinogram on the basis of the scatter sinogram,and reconstructs an image on the basis of the corrected signal sinogram.As described above, according to the present invention, scatteredradiation is removed by a plurality of slice collimators, and theinfluence of scattered components on the signal sinogram is corrected onthe basis of the scatter sinogram. This makes a reconstructed image haveexcellent quantitativeness.

In the PET apparatus according to one aspect of the present invention,each cylindrical detector is characterized by being formed by arrayingtwo-dimensional position detectors, each for detecting thetwo-dimensional incident position of a photon on a light-receivingsurface when the photon is incident on the light-receiving surface, on apredetermined plane in the form of a ring. This arrangement is suitableto improve the resolution of a reconstructed image by reducing the sizeof each photon detection element.

In addition, the PET apparatus according to one aspect of the presentinvention is characterized in that the apparatus further comprisesmoving means for moving the detecting section and the plurality of slicecollimators together relative to an object to be examined which isplaced in the measurement space in a direction parallel to the centralaxis, and the first and second coincidence counting informationaccumulating sections respectively acquire coincidence countinginformation during a period in which the detecting section and theplurality of slice collimators are moved relative to the object by themoving means, convert the coincidence counting information intoinformation in a coordinate system fixed to the object, and accumulatethe information. In this case, coincidence counting information isacquired during a period in which the detecting section and slicecollimators are moved relative to the object in a direction parallel tothe central axis by the moving means. The information is converted intoinformation in a coordinate system fixed to the object and accumulatedin the first coincidence counting information accumulating section orsecond coincidence counting information accumulating section. The imagereconstructing section obtains a reconstructed image on the basis of theaccumulated coincidence counting information (signal sinogram andscatter sinogram). With the above arrangement of the cylindricaldetectors and slice collators, therefore, photon pairs can be detectedwith uniform sensitivity in the body axis direction of the object, andthe quantitativeness of a reconstructed image can be made uniform.

A PET apparatus according to another aspect of the present inventioncomprises a plurality of photon detection elements which are arrangedaround a measurement space and detect one photon and the other photonwhich are produced upon electron-positron pair annihilation, a pluralityof collimators which guide only the photon flying from a predetermineddirection toward each of the plurality of photon detection elements, adetermining section which determines, when detection of one photon byone of the plurality of photon detection elements and detection of theother photon by one of the plurality of photon detection elements aresimultaneously done, whether a straight line connecting alight-receiving surface of the photon detection element which hasdetected one photon and a light-receiving surface of the photondetection element which has detected the other photon crosses any one ofthe plurality of collimators, a first coincidence counting informationaccumulating section which accumulates coincidence counting informationof one photon and the other photon when the determining sectiondetermines that the straight line crosses none of the plurality of slicecollimators, a second coincidence counting information accumulatingsection which accumulates coincidence counting information of one photonand the other photon when the determining section determines that thestraight line crosses one of the plurality of slice collimators, and animage reconstructing section which corrects an influence of a scatteredcomponent on the coincidence counting information accumulated by thefirst coincidence counting information accumulating section on the basisof the coincidence counting information accumulated by the secondcoincidence counting information accumulating section, and reconstructsan image representing a spatial distribution of occurrence frequenciesof a pair of one photon and the other photon in the measurement space onthe basis of the corrected coincidence counting information.

The same as that applies to the PET apparatus according to anotheraspect of the present invention applies to the PET apparatus accordingto one aspect of the present invention.

According to still another aspect of the present invention, an imagegenerating method for a PET apparatus including a plurality of photondetection elements which are arranged around a measurement space anddetect one photon and the other photon which are produced uponelectron-positron pair annihilation, and a plurality of collimatorswhich guide only the photon flying from a predetermined direction towardeach of the plurality of photon detection elements, comprises thedetermining step of determining, when detection of one photon by one ofthe plurality of photon detection elements and detection of the otherphoton by one of the plurality of photon detection elements aresimultaneously done, whether a straight line connecting alight-receiving surface of the photon detection element which hasdetected one photon and a light-receiving surface of the photondetection element which has detected the other photon crosses any one ofthe plurality of collimators, the first coincidence counting informationaccumulating step of accumulating coincidence counting information ofone photon and the other photon when it is determined in the determiningstep that that the straight line crosses none of the plurality of slicecollimators, the second coincidence counting information accumulatingstep of accumulating coincidence counting information of one photon andthe other photon when it is determined in the determining step that thestraight line crosses one of the plurality of slice collimators, and theimage reconstructing step of correcting an influence of a scatteredcomponent on the coincidence counting information accumulated in thefirst coincidence counting information accumulating step on the basis ofthe coincidence counting information accumulated in the secondcoincidence counting information accumulating step, and reconstructingan image representing a spatial distribution of occurrence frequenciesof a pair of one photon and the other photon in the measurement space onthe basis of the corrected coincidence counting information.

The same as that applies to the image generating method for the PETapparatus according to still another aspect of the present inventionapplies to the PET apparatus according to one aspect of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the overall arrangement of a PETapparatus according to the embodiment;

FIG. 2 is a view for explaining the arrangement of the detecting sectionand slice collimator of the PET apparatus according to this embodiment;

FIG. 3 is an enlarged view partly showing the detecting section andslice collimator of the PET apparatus according to this embodiment;

FIG. 4 is a view for explaining the arrangement of cylindrical detectorsand slice collimators of the PET apparatus according to this embodiment;

FIG. 5 is a view showing the arrangement of a block detector mounted inthe PET apparatus according to this embodiment;

FIG. 6 is a view for explaining a signal sinogram S1 and scattersinogram S2 in the PET apparatus according to this embodiment;

FIG. 7 is a view for explaining coincidence counting on a cross-sectiontaken along a plane including the central axis (Z-axis) of the detectingsection mounted in the PET apparatus according to this embodiment;

FIG. 8 is a view for explaining a coincidence counting line when viewedfrom a direction parallel to the central axis (Z-axis) of the detectingsection mounted in the PET apparatus according to this embodiment;

FIG. 9 is a view for explaining accumulation of coincidence countinginformation in the PET apparatus according to this embodiment;

FIG. 10 is a view for explaining the Fourier Rebinning method in the PETapparatus according to this embodiment;

FIG. 11 is a view for explaining the arrangement of the detectingsection of a two-dimensional PET apparatus; and

FIG. 12 is a view for explaining the arrangement of the detectingsection of a three-dimensional PET apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in detail belowwith reference to the accompanying drawings. Note that the samereference numerals denote the same elements throughout the drawings, anda repetitive description will be avoided. Note that a PET apparatus 1according to this embodiment to be described below incorporates all theelements and steps defined in the appended claims.

The arrangement of the PET apparatus 1 according to this embodiment willbe described first with reference to FIGS. 1 to 5. FIG. 1 is a viewshowing the schematic arrangement of the PET apparatus 1 according tothis embodiment. FIG. 1 shows cross-sections of a detecting section 10and slice collimators 21 taken along a plane including a central axisCAX. The PET apparatus 1 according to the embodiment includes thedetecting section 10, slice collimators 21 ₁ to 21 ₁₁, a bed 31, asupport base 32, a moving means 40, a coincidence counting circuit 51, adetermining section 52, a first coincidence counting informationaccumulating section 53, a second coincidence counting informationaccumulating section 54, an image reconstructing section 60, and adisplay section 70. FIG. 1 also shows an object 2 to be examined whichis an examination target for the PET apparatus 1. In addition, a spacewhere the coincidence counting information of photon pairs can bedetected by the PET apparatus 1 is shown as a measurement space 3.

The detecting section 10 has cylindrical detectors 13 ₁ to 13 ₁₂arranged in a direction parallel to the central axis CAX betweenring-like shield collimators 11 and 12. Each cylindrical detector 13_(n) is designed such that a plurality of photon detectors 15 a forrespectively detecting photons flying from the measurement space 3including the central axis CAX are two-dimensionally arranged on acylinder surrounding the central axis CAX. That is, each cylindricaldetector 13 _(n) is equivalent to a unit formed by stacking a pluralityof detector rings, each formed by arranging a plurality of photondetectors 15 a in form of a ring on a plane perpendicular to the centralaxis CAX, in a direction parallel to the central axis CAX. The slicecollimators 21 ₁ to 21 ₁₁ are alternately arranged on the cylindricaldetectors 13 ₁ to 13 ₁₂ in a direction parallel to the central axis CAXat least between the measurement space 3 and the detecting section 10 topass only photons, of the photons flying from the measurement space 3,which are substantially parallel to a predetermined plane toward thedetecting section 10. The detecting section 10 and slice collimators 21₁ to 21 ₁₁ will be described in detail later.

The bed 31 is used to place the object 2 thereon, and supported by thesupport base 32. The moving means 40 moves the detecting section 10 andslice collimators 21 ₁ to 21 ₁₁ together relative to the object 2 placedin the measurement space 3 in a direction parallel to the central axisCAX. More specifically, the moving means 40 may move the detectingsection 10 and slice collimators 21 ₁ to 21 ₁₁ together in a directionparallel to the central axis CAX or may move the bed 31 (i.e., theobject 2) in a direction parallel to the central axis CAX. In addition,the moving means 40 may move them in only one direction duringmeasurement or reciprocate them. Relative movement is done by the movingmeans 40 such that the region of interest of the object 2 is moved by adistance equal to or more than ½ the arrangement pitch of the respectivecylindrical detectors 13 _(n) during measurement. Preferably, the regionof interest of the object 2 is relatively moved at a constant speed inthe measurement space 3 by a distance corresponding to an integermultiple of the above pitch during measurement. If the regions ofinterest of the object 2 exist over a predetermined range in the centralaxis CAX direction, it is preferable that each region in thepredetermined range stay in the measurement space 3 for an almostconstant period of time during measurement.

The first coincidence counting information accumulating section 53 andsecond coincidence counting information accumulating section 54accumulate coincidence counting information of photon pairs detected byone pair of photon detectors 15 a included in the detecting section 10during a period in which the detecting section 10 and slice collimators21 ₁ to 21 ₁₁ are moved relative to the object 2 by the moving means 40.When one pair of photon detectors 15 a included in the detecting section10 simultaneously detect a photon pair, the coincidence counting circuit51 outputs position information indicating the position of each of thepair of photon detectors 15 a.

The determining section 52 determines, on the basis of the positioninformation output from the coincidence counting circuit 51, whether astraight line connecting light-receiving surfaces 15 b of the pair ofphoton detectors 15 a which simultaneously detected the photon paircrosses any one of the slice collimators 21 ₁ to 21 ₁₁. The determiningsection 52 may determine this by the calculation based on thegeometrical structures of each cylindrical detector 13 _(n) and eachslice collimator 21 _(n) or may determine this on the basis of thecoincidence counting information obtained by rotating a rod-likecalibration radiation source without placing the object 2 in themeasurement space 3. In the latter case, the blank data obtained byblank measurement can be used, which is done to correct the sensitivityof the photon detector 15 a.

It is also preferable that a calculation or measurement like thatdescribed above be performed in advance to determine whether a straightline connecting the light-receiving surfaces 15 b of each pair of photondetectors 15 a included in the detecting section 10 crosses any one ofthe slice collimators 21 _(n), and information indicating whether thestraight line crosses any one of the slice collimator 21 _(n) be storedas data “1” or “0” in the form of a table in a ROM, a RAM, or the like.The determining section 52 reads out data from the table by using theposition information output from the coincidence counting circuit 51 asan address, and determines, on the basis of this read data, whether thestraight line connecting the light-receiving surfaces 15 b of the pairof the photon detectors 15 a which have simultaneously detected thephoton pair crosses any one of the slice collimators 21 ₁ to 21 ₁₁.

When the determining section 52 determines that the above straight linecrosses none of the slice collimators 21 ₁ to 21 ₁₁, the firstcoincidence counting information accumulating section 53 accumulates thecoincidence counting information of the photon pair detected by the pairof photon detectors 15 a. The-coincidence counting informationaccumulated in the first coincidence counting information accumulatingsection 53 therefore includes true coincidence counting informationobtained when photons are detected without being scattered andcoincidence counting information obtained when scattered photons aredetected. The coincidence counting information accumulated in the firstcoincidence counting information accumulating section 53 duringmeasurement will be referred to as a signal sinogram S1.

When the determining section 52 determines that the above straight linecrosses any one of the slice collimators 21 ₁ to 21 ₁₁, the secondcoincidence counting information accumulating section 54 accumulates thecoincidence counting information of the photon pair detected by the pairof photon detectors 15 a. The coincidence counting informationaccumulated in the second coincidence counting information accumulatingsection 54 therefore includes no true coincidence counting informationobtained when photons are detected without being scattered, but includesonly the coincidence counting information obtained when scatteredphotons are detected. The coincidence counting information accumulatedin the second coincidence counting information accumulating section 54during measurement will be referred to as a scatter sinogram S2.

In accumulating coincidence counting information in the firstcoincidence counting information accumulating section 53 and secondcoincidence counting information accumulating section 54, thecoincidence counting information detected by the detecting section 10 isconverted into information in a coordinate system fixed to the object 2on the basis of the displacement amounts of the slice collimators 21 ₁to 21 ₁₁ and detecting section 10 relative to the object 2, and thecoincidence counting information having undergone this coordinateconversion is accumulated. Note that as the relative displacementamounts, the data obtained by an encoder or the like or the datarecorded by the moving means 40 may be used.

The light-receiving surface 15 b of each photon detector 15 a includedin the detecting section 10 has a predetermined area. Depending on thepositions on the light-receiving surfaces 15 b of one pair of photondetectors 15 a which are connected by a straight line, the straight linemay cross or may not cross any one of the slice collimators 21 ₁ to 21₁₁. In this case, the coincidence counting information detected by thepair of photon detectors 15 a has an intermediate characteristic betweenthe characteristic of the information accumulated as the signal sinogramS1 and the characteristic of the information accumulated as the scattersinogram S2. It is therefore preferable that such information beaccumulated in one of the first coincidence counting informationaccumulating section 53 and second coincidence counting informationaccumulating section 54 or discarded depending on the degree of thistendency.

The image reconstructing section 60 corrects the influence of scatteredcomponents in the coincidence counting information (signal sinogram S1)accumulated in the first coincidence counting information accumulatingsection 53 on the basis of the coincidence counting information (scattersinogram S2) accumulated in the second coincidence counting informationaccumulating section 54, and reconstructs an image representing thespatial distribution of occurrence frequencies of photon pairs in themeasurement space 3 on the basis of the corrected signal sinogram S1.This image reconstruction processing will be described in detail later.The image reconstructing section 60 also performs sensitivity correctionto correct variations in detection sensitivity of the respective photondetectors 15 a of the detecting section 10 and absorption correction tocorrect the absorption of photons in the object 2. The display section70 displays the reconstructed image obtained by the image reconstructingsection 60. A control section 80 controls relative movement done by themoving means 40, outputting of the position information of a pair ofphoton detectors 15 a by the coincidence counting circuit 51,determination done by the determining section 52, accumulation ofcoincidence counting information by the first coincidence countinginformation accumulating section 53, accumulation of coincidencecounting information by the second coincidence counting informationaccumulating section 54, image reconstruction done by the imagereconstructing section 60, and display of a reconstructed image by thedisplay section 70.

FIG. 2 is a view for explaining the arrangements of the detectingsection 10 and slice collimator 21 of the PET apparatus according tothis embodiment. FIG. 2 shows cross-sections of the detecting section 10and slice collimators 21 ₁ to 21 ₁₁ taken along a plane including thecentral axis CAX. The detecting section 10 of the PET apparatusaccording to this embodiment includes the cylindrical detectors 13 ₁ to13 ₁₂ stacked in a direction parallel to the central axis CAX betweenthe ring-like shield collimators 11 and 12. The respective ring-likeslice collimators 21 ₁ to 21 ₁₁ are located at least between themeasurement space and the detecting section 10 and alternately arrangedon the cylindrical detectors 13 _(n) in a direction parallel to thecentral axis CAX. That is, each slice collimator 21 _(n) is placedbetween the cylindrical detector 13 _(n) and a cylindrical detector 13_(n+1) which are adjacent to each other. Each slice collimator 21 _(n)is made of a material having a larger atomic number and larger specificgravity (e.g., lead or tungsten) and several mm (e.g., 5 mm to 6 mm)thick. Each slice collimator 21 _(n) has a collimating function ofpassing photons, of photons flying from the measurement space, which aresubstantially parallel to a plane perpendicular to the central axis CAXtoward the detecting section 10 and shielding obliquely incidentphotons.

FIG. 3 is an enlarged view of a portion (the portion enclosed with thechain line in FIG. 2) of the detecting section 10 and slice collimators21 of the PET apparatus according to this embodiment. FIG. 4 is a viewfor explaining the arrangement of the cylindrical detector 13 _(n) andslice collimator 21 _(n) of the PET apparatus according to thisembodiment. FIG. 4 shows the relationship between the cylindricaldetector 13 _(n) and the slice collimator 21 _(n) when viewed from adirection parallel to the central axis CAX. Each cylindrical detector 13_(n) has a plurality of block detectors 41 ₁ to 14 _(M) arranged in theform of a ring on the same circumference on a plane perpendicular to thecentral axis CAX. Each block detector 14 _(m) serves as atwo-dimensional position detector for detecting the two-dimensionalincident position of a photon incident on the light-receiving surface 15b. Each slice collimator 21 _(n) reaches the rear portion of thecorresponding cylindrical detector 13 _(n) through the space between thecylindrical detector 13 _(n) and the cylindrical detector 13 _(n+1)which are adjacent to each other and is integrally fixed to a holdingplate 22 at the rear portion.

FIG. 5 is a view showing the arrangement of a block detector 14. Asshown in FIG. 5, each block detector 14 _(m) is a scintillation detectorformed from a combination of a scintillation block 15 constituted by P×Q(P≧2, Q≧2) segments, and a position detection type photomultiplier 16.Each block detector 14 _(m) detects a photon flying from the measurementspace and also detects the two-dimensional incident position of thephoton incident on the light-receiving surface 15 b of the scintillationblock 15. That is, the block detector 14 _(m) is equivalent to a unitobtained by two-dimensionally arranging P×Q small photon detectors 15 a.Each cylindrical detector 13 _(n) constituted by such block detectors 14_(m) arranged in the form of a ring is equivalent to a unit obtained bystacking a plurality of detector rings, each constituted by a pluralityof photon detectors 15 a arranged in the form of a ring on a planeperpendicular to the central axis CAX, in a direction parallel to thecentral axis CAX. In the block detector 14 _(m), a resistor array forapplying a predetermined voltage to each electrode in the positiondetection type photomultiplier 16 and a preamplifier for receiving thecurrent signal output from the anode electrode of the position detectiontype photomultiplier 16 and outputting it as a voltage signal are housedin a casing, together with the scintillation block 15 and positiondetection type photomultiplier 16, for light shielding.

For example, BGO (Bi₄Ge₃O₁₂), GSO (Gd₂SiO₅(Ce)), LSO (Lu₂SiO₅(Ce)), orPWO (PbWO₄) is used for the scintillation block 15, as needed. Thescintillation block 15 is constituted by 8×8 segments, and each segmenthas a size of 6 mm×6 mm×20 mm. The area of the photoelectric surface ofthe position detection type photomultiplier 16 is 50 mm×50 mm. Thecylindrical detector 13 _(n) is formed by arranging 60 block detectors14 _(m), each including the scintillation block 15 and positiondetection type photomultiplier 16, in the form of a ring. Eachcylindrical detector 13 _(n) has an inner diameter of about 1,000 mm.Each slice collimator 21 _(n) has an inner diameter of 600 mm. Thestructure formed by alternately stacking the cylindrical detectors 13 ₁to 13 ₁₂ and slice collimators 21 ₁ to 21 ₁₁ in a direction parallel tothe central axis CAX has a thickness (i.e., the visual field in the bodyaxis direction) of about 670 mm.

Under such conditions, detection of a pair of 511 keV photons (gammarays) generated upon electron-positron pair annihilation in themeasurement space 3 and flying in opposite directions, i.e., detectionof coincidence counting information, may be performed by a pair of blockdetectors 14 in the same cylindrical detector 13 _(n) or a pair of blockdetectors 14 respectively included in the adjacent cylindrical detectors13 _(n) and 13 _(n+1). Detection of coincidence counting information maybe performed by a pair of block detectors 14 included in two separatecylindrical detectors 13. In other words, detection of coincidencecounting information may be performed between two adjacent detectorrings or two separate detector rings as well as within the singledetector ring.

That is, the PET apparatus 1 according to this embodiment has anintermediate arrangement between a conventional two-dimensional PETapparatus and a conventional three-dimensional PET apparatus. Whensensitivity per unit visual field length (cm) in the body axis directionunder the above conditions is calculated, the sensitivity of the PETapparatus 1 according to this embodiment is about 1.3 kcps/(kBq·ml),which is about ½ the sensitivity (about 2.58 kcps/(kBq·ml)) of theconventional three-dimensional PET apparatus, but is about four to fivetimes higher than the sensitivity (about 0.28 kcps/(kBq·ml)) of theconventional two-dimensional PET apparatus.

Each of the coincidence counting circuit 51, determining section 52,first coincidence counting information accumulating section 53, andsecond coincidence counting information accumulating section 54 will bedescribed in further detail next with reference to FIGS. 6 to 9. FIG. 6is a view for explaining the signal sinogram S1 and scatter sinogram S2.Referring to FIG. 6, thin solid lines 1 are coincidence counting linesextending from the light-receiving surface 15 b of a given one of thephoton detectors 15 a as one end, and represent lines indicating theboundaries between coincidence counting information accumulated as thesignal sinogram S1 and coincidence counting information accumulated asthe scatter sinogram S2. The black bullets represent the positions ofelectron-positron pair annihilations. Thick solid lines L represent theflight paths of photons (gamma rays) generated upon electron-positronpair annihilations. The white bullets represent the positions wherephotons are scattered. The broken lines represent the flight paths ofscattered photons.

As shown in FIG. 6, the coincidence counting information of scatteredphotons may be detected by one pair of photon detectors 15 a whosecoincidence counting line crosses one of the slice collimators 21 ₁ to21 ₁₁. In such a case, the corresponding information is determined bythe determining section 52, and the coincidence counting information isaccumulated in the second coincidence counting information accumulatingsection 54, thereby generating the scatter sinogram S2. In addition, thecoincidence counting information of scattered photons may also bedetected, together with the true coincidence counting information ofunscattered photons, by one pair of photon detector 15 a whosecoincidence counting line crosses none of the slice collimators 21 ₁ to21 ₁₁. In this case, the corresponding information is determined by thedetermining section 52, and the coincidence counting information isaccumulated by the first coincidence counting information accumulatingsection 53, thereby generating the signal sinogram S1.

FIG. 7 is a view for explaining coincidence counting at a cross-sectiontaken along a plane including the central axis CAX (Z-axis). Note thatthe illustration of the slice collimator 21 _(n) is omitted in FIG. 7.Referring to FIG. 7, the four thick straight lines with arrows indicateplanes (to be referred to as “projection planes” hereinafter) on whichcoincidence counting information is detected within a single detectorring or between two detector rings, including a projection plane(detector ring difference d=0) on which coincidence counting is donewithin the single detector ring R_(n), a projection plane (detector ringdifference d=1) on which coincidence counting is done between the twodetector rings R_(n) and R_(n+1), a projection plane (detector ringdifference d=2) on which coincidence counting is done between the twodetector rings R_(n) and R_(n+2), and a projection plane (detector ringdifference d=3) on which coincidence counting is done between the twodetector rings R_(n) and R_(n+3). Note that the Z-coordinate of thepoint of intersection between each projection plane and the Z-axis isrepresented by z.

FIG. 8 is a view for explaining a coincidence counting line, on one ofthe above projection planes, viewed from a direction parallel to thecentral axis CAX (Z-axis). Referring to FIG. 8, the line having arrowson its two ends represents a coincidence counting line; q, the azimuthangle in the direction in which coincidence counting is performed; andt, the position coordinate in a direction perpendicular to the directionin which coincidence counting is performed.

As shown in FIGS. 7 and 8, the coincidence counting line is specified bythe distance t from the Z-axis on the projection plane, the azimuthangle q (=0 to 360°) viewed from a direction parallel to the Z-axis, theZ-axis coordinate value z at which the projection plane crosses theZ-axis, and the detector ring difference d (=0, 1, 2, 3, . . . ) When,therefore, one pair of photon detectors 15 a included in the detectingsection 10 simultaneously detect a pair of photons, the coincidencecounting circuit 51 outputs the values of the four parameters t, q, z,and d as position information representing the position of each of thepair of photon detectors 15 a. The determining section 52 determines, onthe basis of the values of the four parameters t, q, z, and d outputfrom the coincidence counting circuit 51, whether a straight lineconnecting the light-receiving surfaces 15 b of the pair of photondetector 15 a which have simultaneously detected the pair of photonscrosses any one of the slice collimators 21 ₁ to 21 ₁₁.

FIG. 9 is a view for explaining accumulation of coincidence countinginformation. As shown in FIG. 9, the first coincidence countinginformation accumulating section 53 accumulates coincidence countinginformation with respect to each of the values of the four parameters t,q, z, and d to generate the signal sinogram S1(t, q, z, d). Likewise,the second coincidence counting information accumulating section 54accumulates coincidence counting information with respect to each of thevalues of the four parameters t, q, z, and d to generate the scattersinogram S2(t, q, z, d). In accumulating coincidence countinginformation in each of the first and second coincidence countinginformation accumulating sections 53 and 54, the coincidence countinginformation expressed in a coordinate system fixed to the object 2 isaccumulated.

Image reconstruction in the image reconstructing section 60 will bedescribed in further detail next. Note that the value of the parameter zis a value in a coordinate system fixed to the object 2 in the followingdescription. First of all, the image reconstructing section 60 acquiresthe signal sinogram S1(t, q, z, d) generated by the first coincidencecounting information accumulating section 53 accumulating coincidencecounting information, and also acquires the scatter sinogram S2(t, q, z,d) generated by the second coincidence counting information accumulatingsection 54 accumulating coincidence counting information.

This scatter sinogram S2(t, q, z, d) exhibits a small change withrespect to the parameter d, and hence the image reconstructing section60 adds the scatter sinograms S2(t, q, z, d) concerning the parameter d.The resultant sinogram is represented by S3(t, q, z, d) in the followingdescription. When the second coincidence counting informationaccumulating section 54 is to accumulate coincidence countinginformation, the section may accumulate coincidence counting informationconcerning the respective values of the three parameters t, q, and z,while neglecting the parameter d, to generate the scatter sinogram S3(t,q, z).

In addition, since this scatter sinogram S3(t, q, z) is sufficientlysmooth with respect to the parameters z and t, the image reconstructingsection 60 preferably smoothen the scatter sinogram S3(t, q, z) withrespect to the parameters z and t by using a low-pass filter (e.g.,Gaussian function filter having a half-value width of about 5 cm). Thiscan reduce statistical noise.

The image reconstructing section 60 then corrects the influence ofscattered components on the signal sinogram S1(t, q, z, d) on the basisof the scatter sinogram S3(t, q, z) obtained in the above manner toobtain a true signal sinogram S(t, q, z, d). More specifically, theimage reconstructing section 60 obtains the true signal sinogram S(t, q,z, d) byS(t,q,z,d)=S 1(t,q,z,d)−k·S 3(t,q,z)·K(d)where K(d) is a sensitivity correction coefficient for correcting thesensitivity of photon detection by a pair of photon detectors 15 a and afunction of the parameter d which is obtained on the basis of the blankdata obtained by the above-described blank measurement. The value of theconstant k is so determined as to minimize the absolute value of thetrue signal sinogram S(t, q, z, d) outside the object 2.

Subsequently, the image reconstructing section 60 reconstructs an imagerepresenting the spatial distribution of occurrence frequencies ofphoton pairs in the measurement space 3 on the basis of the true signalsinogram S(t, q, z, d) obtained in the above manner. As an algorithm forthis image reconstruction, a three-dimensional filter inverse projectionmethod or Fourier Rebinning method is used, as needed. Thethree-dimensional filter inverse projection method is theoretically athree-dimensional extended version of the two-dimensional filter inverseprojection method widely used in X-ray CT and the like. This is a methodof obtaining a reconstructed image by applying appropriatetwo-dimensional filtering to two-dimensional projection data measured invarious projecting directions and then performing three-dimensionalinverse projection of them in the respective directions. In the FourierRebinning method, as shown in FIG. 10, the true signal sinogram S (“(b)”in FIG. 10) corresponding to an inclined projection (“(a)” in FIG. 10)on the detector ring surface is subjected to two-dimensional Fouriertransform with respect to the parameters t and q to obtain atwo-dimensional Fourier transform map (“(c)” in FIG. 10) with respect tovariables n and ω. This two-dimensional Fourier transform map istransformed into two-dimensional Fourier transform maps (“(d)” in FIG.10) of slices parallel to the detector ring surface by using“Frequency-distance relation”, i.e., “γ=−n/ω”. Each parallel slicetwo-dimensional Fourier transform map obtained in this manner issubjected to two-dimensional inverse Fourier transform to obtainprojection data of each parallel slice (“(e)” in FIG. 10). Areconstructed image (“(f)” in FIG. 10) is obtained by performingtwo-dimensional image reconstruction with respect to the projection dataof each parallel slice.

An image generating method in the PET apparatus 1 according to thisembodiment will be described next. The object 2 to which an RI isapplied is placed on the bed 31, and the region of interest of theobject 2 is positioned in the measurement space 3. The PET apparatus 1operates in the following manner under the control of the controlsection 80. First of all, the moving means 40 starts to move thedetecting section 10 and slice collimators 21 ₁ to 21 ₁₁ togetherrelative to the object 2 placed in the measurement space 3 in adirection parallel to the central axis CAX.

Of the 511 keV photons (gamma rays) generated upon electron-positronpair annihilation in the measurement space 3, photons that have reachedthe detecting section 10 without being shielded by the slice collimators21 ₁ to 21 ₁₁ are simultaneously detected by one pair of photondetectors 15 a included in the detecting section 10, and positioninformation indicating the position of each of the pair of photondetectors 15 a is output from the coincidence counting circuit 51. Thedetermining section 52 determines, on the basis of the positioninformation output from the coincidence counting circuit 51, whether astraight line connecting the light-receiving surfaces 15 b of the pairof photon detectors 15 a which have simultaneously detected the photonpair crosses any one of the slice collimators 21 ₁ to 21 ₁₁.

If the determining section 52 determines that the straight line crossesnone of the slice collimators 21 ₁ to 21 ₁₁, the coincidence countinginformation of the photon pair detected by the pair of photon detectors15 a is converted into information in a coordinate system fixed to theobject 2. The resultant information is then accumulated in the firstcoincidence counting information accumulating section 53. If thedetermining section 52 determines that the straight line crosses one ofthe slice collimators 21 ₁ to 21 ₁₁, the coincidence countinginformation of the photon pair detected by the pair of photon detectors15 a is converted into information in the coordinate system fixed to theobject 2. The resultant information is then accumulated in the secondcoincidence counting information accumulating section 54.

When a predetermined measurement period comes to an end, the first andsecond coincidence counting information accumulating sections 53 and 54stop accumulating coincidence counting information, and the moving means40 also stops relative movement. The image reconstructing section 60corrects the influence of scattered components on the signal sinogram S1accumulated and generated by the first coincidence counting informationaccumulating section 53 on the basis of the scatter sinogram S2accumulated and generated by the second coincidence counting informationaccumulating section 54, and reconstructs an image representing thespatial distribution of occurrence frequencies of photon pairs in themeasurement space 3 on the basis of the corrected signal sinogram S1.The image reconstructing section 60 also performs sensitivity correctionand absorption correction. The reconstructed image obtained by the imagereconstructing section 60 is displayed by the display section 70.

As described above, the detecting section 10 of the PET apparatus 1according to this embodiment has the cylindrical detectors 13 ₁ to 13 ₁₂arrayed in a direction parallel to the central axis CAX, and eachcylindrical detector 13 _(n) is formed by two-dimensionally arraying aplurality of photon detectors 15 a on the cylinder surrounding thecentral axis CAX. The cylindrical detectors 13 ₁ to 13 ₁₂ and slicecollimators 21 ₁ to 21 ₁₁ are alternately arranged in a directionparallel to the central axis CAX. With this arrangement of the detectingsection 10, the resolution of a reconstructed image can be improved byreducing the size of each photon detector 15 a. In addition, since theslice collimators 12 _(n) are not arranged between the detector ringsbut are arranged between the cylindrical detectors 13 _(n), intervalsare ensured between the respective slice collimators 12 _(n) to suppressa decrease in open area ratio, thus ensuring high photon pair detectionsensitivity. In addition, since each slice collimator 12 _(n) need notbe thinned, the collimating effect can be maintained, and scatteredradiation can be efficiently removed. This makes it possible to ensurehigh quantitativeness of a reconstructed image. As described above, thePET apparatus 1 according to this embodiment can ensure good photon pairdetection sensitivity and quantitativeness while improving theresolution of a reconstructed image.

In addition, in this embodiment, the determining section 52 determineswhether a straight line connecting the light-receiving surfaces 15 b ofone pair of photon detectors 15 a which have simultaneously detected aphoton pair crosses any one of the slice collimators 21 ₁ to 21 ₁₁, andthe first coincidence counting information accumulating section 53generates the signal sinogram S1 on the basis of this determinationresult. In addition, the second coincidence counting informationaccumulating section 54 generates the scatter sinogram S2 on the basisof the determination result. The image reconstructing section 60 thencorrects the influence of scattered components on the signal sinogram S1on the basis of the scatter sinogram S2, and reconstructs an image onthe basis of the corrected signal sinogram S1. In this manner, thisembodiment corrects the influence of scattered components on the signalsinogram S1 on the basis of the scatter sinogram S2 as well as removingscattered radiation by the slice collimators 21 ₁ to 21 ₁₁, and hencethe quantitativeness of a reconstructed image is excellent.

The scatter correction method according to this embodiment has thefollowing merits as compared with the conventional scatter correctionmethods. According to the first scatter correction method (energy windowmethod), a signal sinogram is obtained by accumulating coincidencecounting information of 511 keV photon pairs, and a scatter sinogram isobtained by accumulating photons with lower energy as components havingundergone Compton scattering in an object to be examined. The product ofthe scatter sinogram and a given constant is then subtracted from thesignal sinogram to obtain a true signal sinogram. The first scattercorrection method is effective in removing scattered radiation fromoutside the visual field in the body axis direction, but requires photondetection elements to have excellent energy resolution characteristics.This method cannot therefore be applied to a case wherein photondetection elements including scintillators that emit only a small amountof light and exhibit low energy resolution are used. In contrast tothis, the scatter correction method in this embodiment can be applied toa case wherein photon detection elements including scintillators thatemit only a small amount of light and exhibit low energy resolution areused, because there is no need to perform energy analysis.

According to the second scatter correction method (a kind of calculationmethod), a scatter response is acquired in advance by placing a pointradiation source in a uniform water phantom having a shape similar tothat of an object to be examined, and a signal sinogram is acquired byplacing the object and measuring it. A pseudo scatter sinogram isobtained by convolution integration of the signal sinogram and scatterresponse, and the scatter sinogram is subtracted from the signalsinogram, thereby obtaining a true signal sinogram. In the secondscatter correction method, scattered components can be removed moreaccurately by repeating the above convolution integration andsubtraction. The second scatter correction method is designed to performcorrection based on a radiation source distribution existing in thevisual field, and hence cannot remove scattered radiation from outsidethe visual field. In contrast to this, in the scatter correction methodin this embodiment, since a scatter sinogram is directly obtained in astate wherein an object to be examined is placed in the measurementspace, a signal sinogram can be accurately corrected by a simplecalculation. In addition, scattered radiation from outside the visualfield can be removed.

According to the third scatter correction method (another kind ofcalculation method), a scatter profile is approximately estimated byinterpolating a scatter profile outside an object to be examined intothe object in consideration of the fact that a portion of a signalsinogram which corresponds to a portion outside the object isconstituted by only scattered components. A true signal sinogram is thenobtained by subtracting the scattered profile from the signal sinogram.The third scatter correction method can approximate a scatter profilerelatively accurately when a radiation source distribution inside theobject is relatively uniform. If, however, radiation sources arelocalized in the object, the scatter profile may have a complicatedshape, which is difficult to estimate. In contrast to this, in thescatter correction method in this embodiment, since a scatter sinogramis directly obtained while an object to be examined is placed in themeasurement space, a signal sinogram can be accurately corrected by asimple calculation, and scattered radiation from outside the visualfield can be removed.

As described above, in the PET apparatus 1 according to this embodiment,the detecting section 10 and slice collimators 21 ₁ to 21 ₁₁ arearranged in the above manner, and scatter correction is performed in theabove manner on the basis of the signal sinogram S1 and scatter sinogramS2 simultaneously acquired during one measurement period. This makes itpossible to ensure good photon pair detection sensitivity andquantitativeness while improving the resolution of a reconstructedimage. In comparison with the conventional methods, the scattercorrection method of this embodiment, in particular, can be applied to acase wherein the photon detectors 15 a including scintillators that emitonly a small amount of light and exhibit poor energy resolution areused, and can accurately correct a signal sinogram with a simple, quickcalculation. In addition, scattered radiation from outside the visualfield can advantageously be removed.

While the detecting section 10 and slice collimators 21 ₁ to 21 ₁₁ aremoved together relative to the object 2 in a direction parallel to thecentral axis CAX by the moving means 40, coincidence countinginformation is accumulated by the first coincidence counting informationaccumulating section 53 or second coincidence counting informationaccumulating section 54, and a reconstructed image is obtained by theimage reconstructing section 60 on the basis of this accumulatedcoincidence counting information (signal sinogram S1 and scattersinogram S2). In this embodiment, therefore, even with the abovearrangement of the cylindrical detectors 13 ₁ to 13 ₁₂ and slicecollimators 21 ₁ to 21 ₁₁, photon pairs can be detected with uniformsensitivity in the body axis direction of the object 2, and thequantitativeness of a reconstructed image can be made uniform.

In addition, in this embodiment, each cylindrical detector 13 _(n) isformed from a ring-like array of a plurality of two-dimensionaldetectors (block detectors 14 _(m)) which detect the two-dimensionalincident positions of photons incident on the light-receiving surfaces15 b. This arrangement is therefore suitable to improve the resolutionof a reconstructed image by reducing the size of each photon detector 15a.

In this embodiment, the slice collimator 21 _(n) reaches the rearportion of each cylindrical detector 13 _(n) through the space betweenthe cylindrical detector 13 _(n) and the cylindrical detector 13_(n +1), and is integrally fixed by the holding plate 22 at the rearportion. In this case, the precision of relative positional relationshipbetween each cylindrical detector 13 _(n) and a corresponding one of theslice collimators 21 _(n) is high, and the respective cylindricaldetectors 13 _(n) and the respective slice collimators 21 _(n) arealternately arranged in a direction parallel to the central axis CAX.This ensures high incidence efficiency of photons on each cylindricaldetector 13 _(n) and sufficiently high performance. In addition, sincethere is no need to strictly manage process accuracy and assemblyaccuracy for the respective cylindrical detectors 13 _(n), slicecollimators 21 _(n), holding plate 22, and the like, the apparatus canbe easily manufactured at low cost. Furthermore, this arrangement issuitable to move the detecting section 10 and slice collimators 21 ₁ to21 ₁₁ together in a direction parallel to the central axis CAX.

INDUSTRIAL APPLICABILITY

As has been described in detail above, detection of coincidence countinginformation may be performed by a pair of photon detection elements in asingle cylindrical detector included in the detecting section, or a pairof photon detection elements respectively included in two adjacentcylindrical detectors depending on the sizes of each cylindricaldetector and each slice collimator, or a pair of photon detectionelements included in two separate cylindrical detectors. In other words,detection of coincidence counting information may be performed betweentwo adjacent detector rings or between two separate detector rings aswell as within the single detector ring. That is, the PET apparatusaccording to the present invention has an intermediate arrangementbetween a conventional two-dimensional PET apparatus and a conventionalthree-dimensional PET apparatus, and has sensitivity about several timeshigher than that of the conventional two-dimensional PET apparatus. ThePET apparatus according to the present invention can therefore ensuregood photon pair detection sensitivity and quantitativeness whileimproving the resolution of a reconstructed image.

In the PET apparatus according to the present invention, in particular,the determining section determines whether a straight line connectingthe light-receiving surfaces of a pair of photon detection elementswhich have simultaneously detected a photon pair crosses any one of theslice collimators. The first coincidence counting informationaccumulating section generates a signal sinogram on the basis of thisdetermination result. The second coincidence counting informationaccumulating section generates a scatter sinogram. The imagereconstructing section then corrects the influence of scatteredcomponents on the signal sinogram on the basis of the scatter sinogram,and reconstructs an image on the basis of the corrected signal sinogram.As described above, according to the present invention, scatteredradiation is removed by a plurality of slice collimators, and theinfluence of scattered components on the signal sinogram is corrected onthe basis of the scatter sinogram. This makes a reconstructed image haveexcellent quantitativeness.

1. A PET apparatus comprising: a detecting section which includes aplurality of cylindrical detectors each formed by two-dimensionallyarraying a plurality of photon detection elements, each of which detectsa photon flying from a measurement space including a central axis, on acylinder surrounding the central axis, the plurality of cylindricaldetectors being arrayed in a direction parallel to the central axis; aplurality of slice collimators which are alternately arranged with saidcylindrical detectors at least between the measurement space and saiddetecting section in a direction parallel to the central axis, and passonly photons, of photons flying from the measurement space, which aresubstantially parallel to a predetermined plane perpendicular to thecentral axis toward said detecting section; a determining section whichdetermines, when a pair of photon detection elements included in saiddetecting section simultaneously detect a photon pair, whether astraight line connecting light-receiving surfaces of the pair of photondetection elements crosses any one of said plurality of slicecollimators; a first coincidence counting information accumulatingsection which accumulates coincidence counting information of the photonpair detected by the pair of photon detection elements when saiddetermining section determines that the straight line crosses none ofsaid plurality of slice collimators; a second coincidence countinginformation accumulating section which accumulates coincidence countinginformation of the photon pair detected by the pair of photon detectionelements when said determining section determines that the straight linecrosses one of said plurality of slice collimators; and an imagereconstructing section which corrects an influence of a scatteredcomponent on the coincidence counting information accumulated by saidfirst coincidence counting information accumulating section on the basisof the coincidence counting information accumulated by said secondcoincidence counting information accumulating section, and reconstructsan image representing a spatial distribution of occurrence frequenciesof photon pairs in the measurement space on the basis of the correctedcoincidence counting information.
 2. A PET apparatus according to claim1, wherein said cylindrical detector is formed by arraying a pluralityof two-dimensional position detectors, each of which detects atwo-dimensional position of a light-receiving surface when a photon isincident thereon, on the predetermined plane in the form of a ring.
 3. APET apparatus according to claim 1, wherein said apparatus furthercomprises moving means for moving said detecting section and saidplurality of slice collimators together relative to an object to beexamined which is placed in the measurement space in a directionparallel to the central axis, and said first and second coincidencecounting information accumulating sections respectively acquirecoincidence counting information during a period in which said detectingsection and said plurality of slice collimators are moved relative tothe object by said moving means, convert the coincidence countinginformation into information in a coordinate system fixed to the object,and accumulate the information.
 4. A PET apparatus comprising: aplurality of photon detection elements which are arranged around ameasurement space and detect one photon and the other photon which areproduced upon electron-positron pair annihilation; a plurality ofcollimators which guide only the photon flying from a predetermineddirection toward each of said plurality of photon detection elements; adetermining section which determines, when detection of said one photonby one of said plurality of photon detection elements and detection ofsaid other photon by one of said plurality of photon detection elementsare simultaneously done, whether a straight line connecting alight-receiving surface of said photon detection element which hasdetected said one photon and a light-receiving surface of said photondetection element which has detected said other photon crosses any oneof said plurality of collimators; a first coincidence countinginformation accumulating section which accumulates coincidence countinginformation of said one photon and said other photon when saiddetermining section determines that the straight line crosses none ofsaid plurality of slice collimators; a second coincidence countinginformation accumulating section which accumulates coincidence countinginformation of said one photon and said other photon when saiddetermining section determines that the straight line crosses one ofsaid plurality of slice collimators; and an image reconstructing sectionwhich corrects an influence of a scattered component on the coincidencecounting information accumulated by said first coincidence countinginformation accumulating section on the basis of the coincidencecounting information accumulated by said second coincidence countinginformation accumulating section, and reconstructs an image representinga spatial distribution of occurrence frequencies of a pair of said onephoton and said other photon in the measurement space on the basis ofthe corrected coincidence counting information.
 5. A PET apparatusaccording to claim 4, wherein said plurality of photon detectionelements are two-dimensionally arrayed to specify positions of thelight-receiving surfaces of said photon detection elements which havedetected said one photon and said other photon.
 6. A PET apparatusaccording to claim 5, wherein the measurement space has a cylindricalshape, and said plurality of photon detection elements aretwo-dimensionally arrayed on a side surface of the cylindrical shape. 7.A PET apparatus according to claim 6, wherein said plurality of photondetection elements are two-dimensionally arrayed to form a plurality ofblock detectors, and said plurality of block detectors are arranged on aside surface of the cylindrical shape in the form of a ring totwo-dimensionally array said plurality of photon detection elements onthe side surface of the cylindrical shape.
 8. A PET apparatus accordingto claim 4, further comprising means for, when said one photon and saidother photon are simultaneously detected, outputting to said determiningsection position information indicating a position of said photondetection element which has detected said one photon and a position ofsaid photon detection element which has detected said other photon.
 9. APET apparatus according to claim 4, further comprising means fordisplaying the image reconstructed by said image reconstructing section.10. An image generating method for a PET apparatus including a pluralityof photon detection elements which are arranged around a measurementspace and detect one photon and the other photon which are produced uponelectron-positron pair annihilation, and a plurality of collimatorswhich guide only the photon flying from a predetermined direction towardeach of the plurality of photon detection elements, comprising: thedetermining step of determining, when detection of said one photon byone of the plurality of photon detection elements and detection of saidother photon by one of the plurality of photon detection elements aresimultaneously done, whether a straight line connecting alight-receiving surface of the photon detection element which hasdetected said one photon and a light-receiving surface of the photondetection element which has detected said other photon crosses any oneof the plurality of collimators; the first coincidence countinginformation accumulating step of accumulating coincidence countinginformation of said one photon and said other photon when it isdetermined in the determining step that that the straight line crossesnone of the plurality of slice collimators; the second coincidencecounting information accumulating step of accumulating coincidencecounting information of said one photon and said other photon when it isdetermined in the determining step that the straight line crosses one ofthe plurality of slice collimators; and the image reconstructing step ofcorrecting an influence of a scattered component on the coincidencecounting information accumulated in the first coincidence countinginformation accumulating step on the basis of the coincidence countinginformation accumulated in the second coincidence counting informationaccumulating step, and reconstructing an image representing a spatialdistribution of occurrence frequencies of a pair of said one photon andsaid other photon in the measurement space on the basis of the correctedcoincidence counting information.
 11. An image generating method for aPET apparatus according to claim 10, wherein the method furthercomprises the step of, when said one photon and said other photon aresimultaneously detected, outputting position information indicating aposition of said photon detection element which has detected said onephoton and a position of said photon detection element which hasdetected said other photon, and the determining step is performed on thebasis of the position information.
 12. An image generating method for aPET apparatus according to claim 10, further comprising the display stepof displaying the image reconstructed in the image reconstructing step.